LHC to re-awaken in 2015 with doubled energy, luminosity

This article, as written by me, appeared in The Hindu on January 10, 2012.

--

After a successful three-year run that saw the discovery of a Higgs-boson-like particle in early 2012, the Large Hadron Collider (LHC) at CERN, near Geneva, Switzerland, will shut down for 18 months for maintenance and upgrades.

This is the first of three long shutdowns, scheduled for 2013, 2017, and 2022. Physicists and engineers will use these breaks to ramp up one of the most sophisticated experiments in history even further.

According to Mirko Pojer, Engineer In-charge, LHC-operations, most of these changes were planned in 2011. They will largely concern fixing known glitches on the ATLAS and CMS particle-detectors. The collider will receive upgrades to increase its collision energy and frequency.

Presently, the LHC smashes two beams, each composed of precisely spaced bunches of protons, at 3.5-4 tera-electron-volts (TeV) per beam.

By 2015, the beam energy will be pushed up to 6.5-7 TeV per beam. Moreover, the bunches which were smashed at intervals of 50 nanoseconds will do so at 25 nanoseconds.

After upgrades, “in terms of performance, the LHC will deliver twice the luminosity,” Dr. Pojer noted in an email to this Correspondent, with reference to the integrated luminosity. Precisely, it is the number of collisions that the LHC can deliver per unit area which the detectors can track.

The instantaneous luminosity, which is the luminosity per second, will be increased to 1x10³⁴ per centimetre-squared per second, ten-times greater than before, and well on its way to peaking at 7.73x10³⁴ per centimetre-squared per second by 2022.

As Steve Myers, CERN’s Director for Accelerators and Technology, announced in December 2012, “More intense beams mean more collisions and a better chance of observing rare phenomena.” One such phenomenon is the appearance of a Higgs-boson-like particle.

The CMS experiment, one of the detectors on the LHC-ring, will receive some new pixel sensors, a technology responsible for tracking the paths of colliding particles. To make use of the impending new luminosity-regime, an extra layer of these advanced sensors will be inserted around a smaller beam pipe.

If results from it are successful, CMS will receive the full unit in late-2016.

In the ATLAS experiment, unlike with CMS which was built with greater luminosities in mind, pixel sensors are foreseen to wear out within one year after upgrades. As an intermediate solution, a new layer of sensors called the B-layer will be inserted within the detector for until 2018.

Because of the risk of radiation damage due to more numerous collisions, specific neutron shields will be fit, according to Phil Allport, ATLAS Upgrade Coordinator.

Both ATLAS and CMS will also receive evaporative cooling systems and new superconducting cables to accommodate the higher performance that will be expected of them in 2015. The other experiments, LHCb and ALICE, will also undergo inspections and upgrades to cope with higher luminosity.

An improved failsafe system will be installed and the existing one upgraded to prevent accidents such as the one in 2008.

Then, an electrical failure damaged 29 magnets and leaked six tonnes of liquid helium into the tunnel, precipitating an eight-month shutdown.

Generally, as Martin Gastal, CMS Experimental Area Manager, explained via email, “All sub-systems will take the opportunity of this shutdown to replace failing parts and increase performance when possible.”

All these changes have been optimised to fulfil the LHC’s future agenda. This includes studying the properties of the newly discovered particle, and looking for signs of new theories of physics like supersymmetry and higher dimensions.

(Special thanks to Achintya Rao, CMS Experiment.)

Gunning for the goddamned: ATLAS results explained

Here are some of the photos from the CERN webcast yesterday (July 4, Wednesday), with an adjoining explanation of the data presented in each one and what it signifies.



This first image shows the data accumulated post-analysis of the diphoton decay mode of the Higgs boson. In simpler terms, physicists first put together all the data they had that resulted from previously known processes. This constituted what's called the background. Then, they looked for signs of any particle that seemed to decay into two energetic photons, or gamma rays, in a specific energy window; in this case, 100-160 GeV.

Finally, knowing how the number of events would vary in a scenario without the Higgs boson, a curve was plotted that fit the data perfectly: the number of events at each energy level v. the energy level at which it was tracked. This way, a bump in the curve during measurement would mean there was a particle previously unaccounted for that was causing an excess of diphoton decay events at a particular energy.



This is the plot of the mass of the particle being looked for (x-axis) versus the confidence level with which it has (or has not, depending n how you look at it) been excluded as an event to focus on. The dotted horizontal line, corresponding to 1μ, marks off a 95% exclusion limit: any events registered above the line can be claimed as having been observed with "more than 95% confidence" (colloquial usage).

Toward the top-right corner of the image are some numbers. 7 TeV and 8 TeV are the values of the total energy going into each collision before and after March, 2012, respectively. The beam energy was driven up to increase the incidence of decay events corresponding to Higgs-boson-like particles, which, given the extremely high energy at which they exist, are viciously short-lived. In experiments that were run between March and July, physicists at CERN reported an increase of almost 25-30% of such events.

The two other numbers indicate the particle accelerator's integrated luminosity. In particle physics, luminosity is measured as the number of particles that can pass detected through a unit of area per second. The integrated luminosity is the same value but measured over a period of time. In the case of the LHC, after the collision energy was vamped up, the luminosity, too, had to be increased: from about 4.7 fb-1 to 5.8 fb-1. You'll want to Wiki the unit of area called barn. Some lighthearted physics talk there.



In this plot, the y-axis on the left shows the chances of error, and the corresponding statistical significance on the right. When the chances of an error stand at 1, the results are not statistically significant at all because every observation is an error! But wait a minute, does that make sense? How can all results be errors? Well, when looking for one particular type of event, any event that is not this event is an error.

Thus, as we move toward the ~125 GeV mark, the number of statistically significant results shoot up drastically. Looking closer, we see two results registered just beyond the 5-sigma mark, where the chances of error are 1 in 3.5 million. This means that if the physicists created just those conditions that resulted in this >5σ (five-sigma) observation 3.5 million times, only once will a random fluctuation play impostor.

Also, notice how the differences between each level of statistical significance increases with increasing significance? For chances of errors: 5σ - 4σ > 4σ - 3σ > ... > 1σ - 0σ. This means that the closer physicists get to a discovery, the exponentially more precise they must be!



OK, this is a graph showing the mass-distribution for the four-lepton decay mode, referred to as a channel by those working on the ATLAS and CMS collaborations (because there are separate channels of data-taking for each decay-mode). The plotting parameters are the same as in the first plot in this post except for the scale of the x-axis, which goes all the way from 0 to 250 GeV. Now, between 120 GeV and 130 GeV, there is an excess of events (light blue). Physicists know it is an excess and not at par with expectations because theoretical calculations made after discounting a Higgs-boson-like decay event show that, in that 10 GeV, only around 5.3 events are to be expected, as opposed to the 13 that turned up.

Understanding accelerator luminosity

Advanced physics is essentially a study in precision, and the particle accelerators of today that are located at the cutting-edge Intensity and Energy Frontiers work against approximations everyday. The particles they synthesize, track and study are so small, quick and short-lived that they might as well have simply popped in and out of existence and nothing would've changed. However, fortunately, that's not the point of studying these things at all: understanding why the "popping" happens at all is what is key.

[caption id="attachment_21639" align="aligncenter" width="346" caption="Some famous accelerators: (clockwise from top-left) Kō Enerugī Kasokuki Kenkyū Kikō (KEK), Japan; Tevatron at FERMILAB; CERN's Large Hadron Collider; and LINAC at Stanford Linear Accelerator Centre."][/caption]

At the world's most powerful collider, the LHC at CERN, two proton beams are shot around 27-km long rings. These are not continuous beams but ones intermittently segregated into bunches, like a pulse. Each of these bunches contains 2,808 protons (which are the hadrons in question) and there are 1,000 bunches per beam. It is ensured that the bunches from the rings don't cross each other - "collide" - more than once every 25 nanoseconds. At this rate, 112.32 billion protons - 56.16 billion from each side - meet each other every second. This is what every particle accelerator makes possible: a rendezvous.

Once this is done, the detectors take over, and they are the real measure of an accelerator's performance. The accelerator will have ensured that enough collisions occur so that the detector can record at least one (even though I'm understating the ratio, it is really quite small). Ergo, to measure a detector's performance as either being good or bad, or perhaps even as somewhere in between in the rare case, how much it is capable of seeing is what makes the difference. This is where luminosity comes in.

The generic definition of luminosity is that it is a measure of the quantity of light that passes through an area each second, and so its units are per metre-squared per second. Accelerator physics adopted this definition and modified it a little: accelerator luminosity is a measure of the number of particles that pass through a given area each second multiplied by the opacity of the detector. This final parameter is necessary because it also accounts for the tendency of some particles to escape detection by passing right through the target: if the target's opacity is high, most particles will be "seen", and if it is low, most particles will be invisible to the cameras' eyes.

(Even though the definition of luminosity indicates the number of particles that pass through an area per second, its meaning in the confines of an accelerator changes: it is the number particles that are seen by a detector irrespective of how many particles there are in total.)

Inside the accelerator and in the presence of the detector, the following differential equation dictates the machine's luminosity:



Here, σ is the total cross section of the detector - the area that is exposed to and receives the stream of particles, N the number of particles, L the instantaneous luminosity, and t the duration over which the detector remains in operation. The opacity affects σ. (The 'd' denotes that the value of the parameter is being considered for an infinitesimal period of time, as indicated by the dt in the denominator. If it was dx or dy instead of dt, it would mean the value of N is being considered over a very small distance in the x or y direction.)

If Ω (omega) were the solid angle through which the detector's cross section was exposed, its differential cross section is computed as



 

This formula gives the luminosity with respect to the angular cross section (as opposed to a planar surface) as the number of particles per degree per second, and from here, the number of particles per volume of space can be easily computed. The formula also shows that the greater the detecting cross section per degree of solid angle, the greater the luminosity per degree of the same angle (or, "particle-seeability"). And for the detector to be useful at all, the instantaneous luminosity has to be high enough to detect particles so small that... well, they're incredibly small. Therefore, the smaller the particle being studied, the larger the detector will be.

There is no better way to illustrate this conclusion than to point, again, to the LHC, where the Higgs boson particle, one of the smallest particles conceivable, a veritable building block of nature, is being hunted by the world's largest detector (which also has a misleading name): the Compact Muon Solenoid (CMS). The CMS, weighing 12,500 tons, has been able to achieve an astounding integrated (as in not instantaneous) luminosity of 1 per femtobarn: 1 barn is one-hundred-billion-billion-billionth of a squared metre; 1 femtobarn is one-million-billionth of that!

[caption id="attachment_21633" align="aligncenter" width="461" caption="The total integrated luminosity delivered to and collected by CMS until 17th June, 2011."][/caption]

Another detector at the site, the much more prolific A Toroidal LHC Apparatus (ATLAS) weighs 7,000 tons and has a luminosity of 50 per femtobarn. The under-construction iron-calorimeter (ICAL) detector at the India-based Neutrino Observatory (INO) in Theni, Tamil Nadu, will weigh 50,000 tons after being completed in 2015 and will be used to track and study neutrinos exclusively. Neutrinos are particles more elusive than the Higgs, and, though the luminosity of ICAL hasn't been disclosed, we can expect the device to be one of the pioneers in detector technology simply because its luminosity must be that low for the project to be a success.

This much and more can be said of accelerator luminosity. While the media goes gaga over the energies at which the beams are being accelerated, there is a silent revolution in detector technology happening in the background, a revolution that is spawning brilliant techniques to spot the fastest, smallest and most volatile particles. These detectors also consume the greater part of accelerator budgets to build and the greater part of total maintenance time. Some of the most advanced detectors in existence include hadronic calorimeters (HCAL), ring-imaging Cherenkov detectors (RICH detectors) and muon spectrometers.